This invention is generally related to thermally insulating windows, and more specifically to a high thermal efficiency, sealed thermal pane window containing a gas or vapor at moderately low vacuum.
Undesired thermal losses and gains through windows have been estimated to account for five percent of the total energy consumption in the United States. Most windows in the U.S. are old, energy-inefficient single and double pane units having insulating values of about R-1 and R-2 respectively. (R-value is a standard unit indicating resistance to heat transfer in terms of .degree.F-ft.sup.2 -hr/BTU.) By contrast, exterior walls designed to current energy efficiency standards yield R-values from R-11 to R-30. Consequently, considerable effort is being devoted worldwide to the development of high performance window glazings.
One such effort involves the continued development of low-emissivity infrared reflective coatings which are applied to an interior surface of a double pane window. These low emissivity windows are currently available in the market and increase the thermal effectiveness of double pane windows from R-2 to about R-3 at a moderate increase in cost. A more advanced window design described in U.S. Pat. No. 4,335,166, issued to R. A. Lizardo and R. D. O'Shaughnessey, incorporates a low emissivity coating on a clear plastic film suspended between two panes of glass. This design is reported to achieve thermal insulating values of R-4 when filled with air, and as high as R-5 when filled with argon. Although an improvement, these low emissivity windows are still thermally inefficient when compared with the walls surrounding the windows.
Another effort to develop high performance window glazings involves the use of very low pressure vacuum in a sealed dual pane unit. U.S. Pat. No. 4,683,154, issued to D. K. Benson and C. E. Tracy, describes a window evacuated to a pressure of about 10.sup.-6 torr (760 torr=1 atmosphere) and using a low emissivity coating on one or both of the interior surfaces resulting in an estimated thermal insulating value of R-10 to R-12. Since conventional window edge seals are ineffective at preventing the loss of the extreme vacuum, Benson and Tracy propose an all glass welded edge seal fabricated by heating the two panes of glass to above their annealing temperature (about 1000.degree. F.) in a vacuum furnace, allowing the edges to deform slightly, and using a laser beam to weld the edges together.
Although the application of very low pressure vacuum in dual pane windows as described in U.S. Pat. No. 4,683,154 appears promising, significant practical obstacles remain. The requirement that the glass panes be heated to above their annealing point is both costly and effectively precludes the use of laminated safety glass or tempered glass to provide a margin of safety in the event of glass breakage. Further, many low emissivity, reflective and specialty coatings are damaged by high temperatures. Maintenance of the extreme vacuum (about one-billionth of atmospheric pressure) is a formidable obstacle which requires the use of a reactive metal getter to absorb outgassed and transmitted reactive gases. Finally, the highly conductive edge seals transfer enough heat to degrade the overall window performance by as much as 30% unless a bulky insulated edge baffle is used.